1. Field of the Invention
This invention relates generally to CMOS imaging devices, and more particularly to a device and method for correcting lighting artifacts caused by transient light, such as light strobes or flashes, in images captured by CMOS imagers.
2. Description of the Related Art
CMOS imagers are on form of light sensors or transducers that convert optical images into electric signals for digital storage or transmission. CMOS imagers are rapidly becoming the preferred imager over CCD for video and still digital cameras of all resolutions. CMOS devices are preferred over CCD devices largely due to their benefits over CCD devices with respect to cost, ease of manufacture, lower power consumption, and lower heat.
Conventional CMOS imagers provide a sequential output, whereby pixels are read from top to bottom of the device, line by line. In this sense, a CMOS imager operates similar to a flatbed scanner, reading a line at a time. This behavior makes the CMOS susceptible to time sensitive disturbances. For example, when using a conventional CMOS imaging device, a quick luminance disturbance may only appear on the portion of a frame that corresponds to the pixel rows that were captured during the time of the disturbance. This means that quick disturbances, such as strobe lights or flashes, will only be visible in a portion of a given frame, or may begin in the middle of one frame and end in the middle of another frame.
This phenomenon is not visible in images taken with CCD devices. CCD imagers capture and output a complete image at a time. Therefore, in CCD imagers, a sudden luminance disturbance, such as a flash, affects a whole frame or picture at a time.
This image artifact or disturbance, whereby a flash only affects a portion of a frame, has been accepted by the professional users of CMOS imaging devices. However, end-users object to this artifact. Most end-users are more familiar with flash used in conjunction with CCD imagers where, when taking pictures, the entire frame brightens. Therefore, end-users have been neither comfortable nor familiar with the behavior of CMOS imaging devices. Software exists that addresses this behavior in video in post-production. The software eliminates the artifacts by comparing the previous frame or the following frame with the current frame, and then replacing the brightened frame with the previous or following frame. As such, multiple flashes result in picture freezing. This introduces a whole new set of motion artifacts due to the variance between the current frame and the previous or following frame.
FIG. 1 illustrates video frames taken at a car accident scene using a conventional CMOS imaging device. Frames 1-4 and 6-8 illustrate the same general luminance levels. Frame 5, however, illustrates an image taken in conjunction with a quick luminance disturbance, such as a flash, light strobe, or lightening strike. Since the luminance disturbance is quick, it only affects a portion 10 of frame 5 due to the sequential operating nature of the CMOS. Since the luminance disturbance is shorter than the time required to capture all the lines in the frame, the artifact caused by the luminance disturbance only appears in a portion 10 of the frame.
As explained above, CMOS luminance disturbances are not limited to only a single frame. A luminance disturbance may span more than one frame. In such a case, the first and last frames in the span will have one portion appear at regular luminance levels while the rest of the frame may have heightened luminance levels.
FIG. 2 illustrates a timing diagram of the luminance levels of the pixels from the conventional CMOS imaging device captured in frame 1-8 of FIG. 1. The recorded luminance levels for frames 1-4 are all similar, as are the luminance levels of frames 6-8. However, the timing diagram for frame 5 illustrates the effect 40 (shaded) of the luminance disturbance on the recorded luminance levels. The beginning of the luminance disturbance causes a large, sudden gain 20 in the luminance levels, and the end of the luminance disturbance causes a large, sudden drop 30 in the luminance levels.
There is currently no compensation for this CMOS artifact/phenomenon in any cameras. As such, absent software correction, images captured by CMOS imaging devices may show frames where a portion of the image is brightened by the luminance disturbance.